This young lady vanished into thin air. You know that was the intriguing part. That was the piece of the puzzle that you had to solve first. Reporter: Private investigator ken Brennan believes a big piece of that puzzle lies with this big guy. He's seen entering the elevator with inn at 3:41 A.M., then exiting the hotel with a suitcase at 5:28. About an hour later, he comes back and he re-enters the hotel, but he doesn't have the suitcase with him. That suitcase that he brought out earlier comes back a little over an hour later without the suitcase. Reporter: Huh. Does that register something right away in your mind? As soon as he comes back without the suitcase, now he becomes a person of interest to me. He was the only one that had the time and the opportunity to commit the crime and come back to the scene. Reporter: Now upon closer and repeated examination, Brennan returns to this crucial moment. When the man exits the elevator and gives the suitcase a tell-tale tug. The suspect has to reach back and grab the handles with both hands to pull that suitcase out of the crack. I do a lot of traveling, you do a lot of traveling, I'm sure you have a lot of your clothing items in there. Was it ever so heavy you needed two hands to pull it out of the crack? No, probably not. Soon as he tugged that with both hands, like, a light bulb went off in my head, I said, "This is my guy. This is my guy." She's in that suitcase. Reporter: But that light bulb was only over Brennan's head. You know, where's this guy coming out of left field with this black, male suspect? Reporter: Nobody believed a grown woman could fit into that little suitcase. But this is a small suitcase. What would lead you to even think a woman could fit into a small suitcase? Well, the reason why I thought a woman might be able to fit into the suitcase is because this is a large guy. So our frame of reference on how large the suitcase is might be a little off. So, I decided to use a measurement of the suitcase in relationship with markings on the elevator itself. Reporter: With that reference point, incredibly, Brennan's able to determine the bag is actually 29 inches high. And examining this image, where we see the suitcase against this floor runner, he concludes the bag is 19 inches wide, just like this one. Then I knew exactly how wide the suitcase was. Reporter: Oh. And then you were able to really theorize that someone could fit in there. That's correct. What I did was, had one of the fellas that works for me had a girlfriend who was that size. So we got ahold of a suitcase that was the same dimensions and see if she could fit in it. Reporter: She did? Yeah, she can, she fits fine. Reporter: If you think that sounds a bit bizarre, we thought so too. So, we went to a local gymnastics school and asked these two instructors to help us. This woman stands 5'3", just like inn. And sure enough, she fits comfort Bly inside the suitcase and gets zipped in by her friend. She'll be okay. Then, she gets wheeled around the gym, emerging no worse for wear. So I knew that she would fit. Alls I was doing was basically validating what my hunch was. Reporter: Brennan has now deduced the one theory of the case which matches all the evidence. This man attacked inn, sometime after they entered the elevator together. He dragged her body out of the hotel in this suitcase, drove off with her, dumped her body at that vacant lot, then returned. All the puzzles started fitting together at that particular point. I knew I had my guy. And I just had to find out who the heck he was. Reporter: But it's still too early for high fives. Isolating the perpetrator on video is one thing. Catching him, another. And for ken Brennan, that job is quickly becoming an obsession. I said hell or high water, I know I'm right. I can feel it. I knew this was the guy. And I was going to catch this guy.

This transcript has been automatically generated and may not be 100% accurate.

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